The present invention relates to a photographing mode display switching circuit for a camera in which mechanical switches operating in association with the shutter dial, the apperture ring and the shutter speed setting push bottom are operated to select among various photographing mode displays.
In general, the states of use of the electronic circuitry in a camera can be divided into a photometric state and a control state. The term "photometric state" as used herein is intended to mean a state in which photometric parameters, such as the viewed object's brightness, film sensitivity, aperture value and shutter speed, are calculated with the photometric switch closed. The term "control state" as used here is intended to mean a state in which the aperture or the shutter is controlled during the release operation.
Recently, electronic circuitry for cameras has been remarkably digitized. The clock pulse generating source for such circuitry employs a crystal. However, because considerable time is required for the oscillation of a crystal to stabilize, the oscillation circuit must be in operation at all times. The amount of current required by the circuit in the standby state is generally on the order of several microamperes.
The conventional electronic circuit for a camera is constructed as shown in FIGS. 1 and 2. In FIG. 1, reference character E designates a battery for supplying current to a first circuit block A. A clock pulse generating crystal CR is connected to the first circuit block A. The first circuit block A has connected thereto a plurality of mechanical switches S.sub.1, S.sub.2, . . . and S.sub.n on its input side, each having one terminal connected to ground, and an extremely low power consumption type display element, which may for instance be a liquid crystal display element. The battery is connected through a photometric switch MS to a second circuit block B. The first and second circuit blocks A and B are connected together through signal lines DB-A and DB-B. Signals from the first circuit block A are transmitted to the second circuit block B through the signal line DB-A. Signals from the second circuit block B are transmitted through the signal line DB-B to the first circuit block A.
The first circuit block A is employed to monitor the states of the mechanical switches S.sub.1 through S.sub.n and to display the monitored states on the display element DP. The mechanical switches S.sub.1 through S.sub.n are operated in association with the operations of the shutter dial, the aperture ring, a push button used for setting the shutter speed, etc. The states of the switches S.sub.1 through S.sub.n select among various photographing modes, such as a shutter priority mode, an aperture priority mode and a manual mode, and an indication of the selected mode is displayed on the display element DP at all times. If a mechanical switch is provided which is operated in association with the shutter speed setting push button, when selecting the shutter priority mode or the manual priority mode, the photographer can set the shutter speed while observing the display element DP with the photometric switch MS maintained open.
The photographing mode and the setting of the shutter speed are applied from the first circuit block A through the signal line DB-A to the second circuit block B. Upon closure of the photometric switch MS, the second circuit block receives the data from the first circuit block needed to perform the photometric calculations. When necessary, the results of such calculations are applied through the signal line DB-B to the first circuit block A and displayed on the display element DP.
The first circuit block A is shown in FIG. 2 in more detail. The mechanical switches S.sub.1 through S.sub.n are connected in series with resistors r.sub.1 through r.sub.n, respectively, The output signals of the switches at the junction points of the switches and the resistors are applied to a logic operation circuit LC. The series circits of the switches and the resistors are connected between the positive terminal V.sub.DD and the negative terminal GND of the battery E.
A specific example of the conventional circuit will now be discussed. If the voltage of the battery E is 3 V, the number n of mechanical switches is five, and the resistance of each of the resistors r.sub.1 through r.sub.n is 1 megohm, the magnitude of the current flowing in a closed mechanical switch is 3 .mu.A. Accordingly, if all five of the switches are closed, the total of the currents flowing in these switches is 15 .mu.A, a sufficiently high value that the battery will be consumed quite rapidly.
Thus, in the conventional circuit, the life of the battery is unavoidably short, even if the resistances of the resistors r.sub.1 through r.sub.n are set to an unpractical value of the order of several megohms.